Magnetic media are widely used in various applications, particularly in the computer industry, and efforts are continually made with the aim of increasing the areal recording density, i.e., bit density of the magnetic media. In this regard, so-called “perpendicular” recording media have been found to be superior to the more conventional “longitudinal” media in achieving very high bit densities. In perpendicular magnetic recording media, residual magnetization is formed in a direction perpendicular to the surface of the magnetic medium, typically a layer of a magnetic material on a suitable substrate. Very high linear recording densities are obtainable by utilizing a “single-pole” magnetic transducer or “head” with such perpendicular magnetic media.
It is well-known that efficient, high bit density recording utilizing a perpendicular magnetic medium requires interposition of a relatively thick (i.e., as compared to the magnetic recording layer), magnetically “soft” underlayer (“SUL”), i.e., a magnetic layer having relatively low coercivity, such as of a NiFe alloy (Permalloy), between the non-magnetic substrate, e.g., of glass, aluminum (Al) or an Al-based alloy, and the “hard” magnetic recording layer, e.g., of a cobalt-based alloy (e.g., a Co—Cr alloy) having perpendicular anisotropy or of a (CoX/Pd or Pt)n, multi-layer superlattice structure. The magnetically soft underlayer serves to guide magnetic flux emanating from the head through the magnetically hard, perpendicular magnetic recording layer. In addition, the magnetically soft underlayer reduces susceptibility of the medium to thermally-activated magnetization reversal by reducing the demagnetizing fields which lower the energy barrier that maintains the current state of magnetization.
A typical perpendicular recording system 10 utilizing a vertically oriented magnetic medium 1 with a relatively thick soft magnetic underlayer, a relatively thin hard magnetic recording layer, and a single-pole head, is illustrated in FIG. 1, wherein reference numerals 2, 3, 4, and 5, respectively, indicate the substrate, soft magnetic underlayer, at least one non-magnetic interlayer, and vertically oriented, hard magnetic recording layer of perpendicular magnetic medium 1, and reference numerals 7 and 8, respectively, indicate the single and auxiliary poles of single-pole magnetic transducer head 6. Relatively thin interlayer 4 (also referred to as an “intermediate” layer), comprised of one or more layers of non-magnetic materials, illustratively a pair of layers 4A and 4B, is provided in a thickness sufficient to prevent (i.e. de-couple) magnetic interaction between the soft underlayer 3 and the hard recording layer 5 but should be as thin as possible in order to minimize the spacing HSS between the lower edge of the transducer head 6 and the upper edge of the magnetically soft underlayer 3. Spacing HMS between the lower edge of the transducer head 6 and the upper edge of the hard magnetic recording layer 5 is also minimized during operation of system 10. In addition to the above, interlayer 4 also serves to promote desired microstructural and magnetic properties of the hard recording layer 5.
As shown by the arrows in the figure indicating the path of the magnetic flux φ, flux φ is seen as emanating from single pole 7 of single-pole magnetic transducer head 6, entering and passing through vertically oriented, hard magnetic recording layer 5 in the region above single pole 7, entering and travelling along soft magnetic underlayer 3 for a distance, and then exiting therefrom and passing through vertically oriented, hard magnetic recording layer 5 in the region above auxiliary pole 8 of single-pole magnetic transducer head 6. The direction of movement of perpendicular magnetic medium 1 past transducer head 6 is indicated in the figure by the arrow above medium 1.
With continued reference to FIG. 1, vertical lines 9 indicate grain boundaries of each polycrystalline (i.e., granular) layer of the layer stack constituting medium 1. As apparent from the figure, the width of the grains (as measured in a horizontal direction) of each of the polycrystalline layers constituting the layer stack of the medium is substantially the same, i.e., each overlying layer replicates the grain width of the underlying layer. Completing medium 1 are a protective overcoat layer 11, such as a layer of diamond-like carbon (DLC) formed over hard magnetic layer 5, and a lubricant topcoat layer 12, such as a layer of a perfluoropolyethylene material, formed over the protective overcoat layer 11. Substrate 2 is typically disk-shaped and comprised of a non-magnetic metal or alloy, e.g., Al or an Al-based alloy, such as Al—Mg having an Ni—P plating layer on the deposition surface thereof, or substrate 2 is comprised of a suitable glass, ceramic, glass-ceramic, polymeric material, or a composite or laminate of these materials and may include an adhesion layer 2A at the upper surface thereof, typically comprised of an about 10 to about 50 Å thick layer of Cr; soft magnetic underlayer 3 is typically comprised of an about 1,000 to about 4,000 Å thick layer of a soft magnetic material selected from the group consisting of Ni, NiFe (Permalloy), Co, CoFe, Fe, FeN, FeSiAl, FeSiAlN, etc.; the at least one interlayer 4 typically comprises a layer or a pair of up to about 10 Å thick layers 4A, 4B, of at least one non-magnetic material, such as Pt, Pd, Ta, Ru, Ti, Ti—Cr, and Co-based alloys; and hard magnetic layer 5 is typically comprised of an about 60 to about 300 Å thick layer of a Co-based alloy including one or more elements selected from the group consisting of Cr, Fe, Ta, Ni, Mo, Pt, V, Nb, Ge, and B, iron oxides, such as Fe3O4 and δ-Fe2O3, or a (CoX/Pd or Pt)n multilayer magnetic superlattice structure, where n is an integer from about 10 to about 25, each of the alternating, thin layers of Co-based magnetic alloy is from about 2 to about 3.5 Å thick, X is an element selected from the group consisting of Cr, Ta, B, Mo, and Pt, and each of the alternating thin, non-magnetic layers of Pd or Pt is about 10 Å thick. Each type of hard magnetic recording layer material has perpendicular anisotropy arising from magneto-crystalline anisotropy (1st type) and/or interfacial anisotropy (2nd type).
Currently, no existing technique based upon utilization of a conventionally employed Vibrating Sample Magnetometer (“VSM”) is available for accurately measuring the magnetic remanence-thickness product (Mrt), hereinafter “sample perpendicular magnetic moment”, and magnetic anisotropy field strength (Hk), of such perpendicular recording media comprising a magnetically soft underlayer (SUL), due to the overwhelming contribution from the SUL to the VSM signal. As a consequence, an optical method relying upon measurement of the Kerr effect and termed “SMOKE”, is frequently employed as an alternative method for measuring M-H loops of SUL-containing perpendicular media, which SMOKE method is incapable of providing absolute measurements of Mrt.
In view of the above, there exists a clear need for improved methodology for measuring absolute values of the magnetic moment Mrt, as well as anisotropy field Hk, of SUL-containing perpendicular magnetic recording media. In addition, there exists a need for improved methodology for rapidly, accurately, and cost-effectively performing measurements of absolute Mrt anf Hk values of SUL-containing, high areal recording density perpendicular magnetic recording media, e.g., hard disks.
The present invention addresses and solves problems attendant upon the measurement of absolute Mrt and Hk values of SUL-containing high areal recording density perpendicular magnetic recording media, while maintaining full compatibility with all structural and mechanical aspects of high bit recording density technology. Moreover, the methodology according to the present invention can be practiced by means of conventional apparatus and instrumentalities.